A common approach to gas turbine engine shutdown is to reduce the flow of fuel gradually over time. Once the flow of fuel and/or the rotor speed are sufficiently low for a particular turbine, the fuel flow may be stopped when the turbine decelerates to a minimum speed. This minimum speed may be known as the “turning gear speed”, i.e., the speed at which the rotor must be continually turned by an outside source so as to prevent thermal bowing of the rotor. Depending upon the nature of the gas turbine engine, the fuel flow may be stopped at about twenty percent (20%) or so of typical full speed with the turning gear engaged at about one percent (1%) or so of full speed.
Reducing the flow of fuel flow over time, however, does not provide a direct relationship with the speed of the rotor. Rather, large variations in shutdown behavior may result. For example, variations in the speed of the rotor verses time may result. These variations in the speed of the rotor may produce significant differences in the fuel to air ratio because air intake is a function of the speed of the rotor while fuel flow is not directly related to speed. Varied shutdown times thus may lead to variations in firing temperatures, transient temperature gradients, variations in rotational speed, variations in emissions, and the like. Moreover, the variations in shutdown behavior may have an impact on turbine clearances and, hence, overall turbine performance and component lifetime.
There is therefore a desire for improved systems and methods for optimized gas turbine shutdown. Preferably, these improved systems and methods may control the rate of deceleration of the turbine rotor and related components over time so as to minimize shutdown variations and the impact thereof in a dynamic open loop fashion.